The ability to transport fluids in micron-sized channels is essential for many emerging technologies, such as in vivo drug delivery devices, micro-electro-mechanical systems (MEMS), and micro-total-analysis systems (μTAS). New methods for the rapid mixing of non-homogeneous fluids in micron-scale devices are also required, since the absence of turbulent mixing on these small length scales implies that mixing occurs by molecular diffusion alone. This typically takes from seconds to minutes—far too slow for envisioned applications. New technologies are thus required for the manipulation, transport and mixing of fluids on these small length scales.
Microfluidics is a growing area of science and technology with important applications in biomedical devices and portable electronics. Traditional pressure-driven flows do not scale well with miniaturization, due to large viscous stresses, so other pumping techniques have been explored. An attractive alternative is electro-osmosis, the effective slip of a liquid electrolyte past a solid surface in response to an applied electric field, since it does not involve any moving parts, is unaffected by miniaturization, and integrates well with standard microelectronics and fabrication methods. The standard technique of (capillary) electro-osmosis involves a DC electric field applied down a microchannel made of insulating material to generate a plug flow. The electric field acts on the equilibrium surface charge in the diffuse-part of the double layer, and the resulting electro-osmotic flow is linear in the applied field.
Various methods have been described to alter the surface charge in linear electro-osmosis to allow some degree of local flow control. For example, one method applies “field-effect electro-osmosis” to control capillary electro-osmosis by applying voltages at secondary electrodes just outside the channel surface, to alter the equilibrium surface charge (or “zeta potential”) driving steady flow at the insulating channel wall and another controls liquid flow down a traditional insulating capillary by the same effect.
Capillary electro-osmosis, with or without field-effect flow control, however, is not ideal for certain microfluidic applications, since the electric field is applied down the channel, a large voltage is required, e.g. 100 Volts across a 1 cm device to generate typical fields of 100 V/cm. Since electro-osmosis is linear in the applied field, a direct current must be sustained through Faradaic electrochemical reactions at the electrodes generating the field, which can produce gas bubbles, electrode degradation/dissolution, hydrodynamic instability, and sample contamination. Further, the typical fluid velocity is fairly small (e.g. 100 micron/sec for a 100 Volt/cm field) and only increases linearly with the voltage. It would be preferable to drive flows with low-voltage alternating currents in many microfluidic applications, while somehow increasing the flow rate for the same applied field.
Microfluidic devices based on nonlinear electro-osmotic flow have also been developed. For example, nonlinear electro-osmotic flow, varying as the square of the applied voltage, termed “AC electro-osmosis” (ACEO), over a pair of flat, parallel-stripe microelectrodes on a flat insulating surface, has been described.
While existing ACEO pumps operate at much lower power (mA) and lower voltage (Volt) than microfluidic pumps based on linear electro-osmosis, nonetheless, current ACEO devices are somewhat inefficient for long-range pumping, and more general flow topologies for simultaneous mixing and pumping have not been developed.